SmedSpeed XS650


750 and 880cc big bore engine kits

The Yamaha XS650 responds very well to an increase in cubic capacity, and the resultant increase in compression ratio, yielding an improved torque output at lower rpm. It is no real surprise as Yamaha designed in the facility to enlarge the engine to 750 cc at the paper stage. The 1972 model year saw a batch of 200 XS2 standard bikes, all came from the factory with 750 pistons. This special batch was required to meet AMA homologation rules, regarding the use of “stock” size engines for flat track racing. These engines were all engine numbered GS650 instead of the normal S650 engine number for the year.

750cc big bore engine
Wiseco had changed their 750 big bore piston ordering policy, making it hard to order easily in the UK, however in deference to the unbelievable popularity of a an obsolete engine, they have continued production. I think it has a lot more to do with balance sheets than philanthropy, but that I suppose is business. I have recently had 750 pistons from JE in the USA. They have a much higher dome shape, not unlike the old Triumph high compression pistons, the dome shape is a bit of a compromise, but if you need 11:1 compression its the only easy way to do it.  

750cc big bore engine

Standard cylinders can have their cylinder liners removed and after a lathe boring procedure to within 0.002 (imperial) using a SmedSpeed designed tool, the cylinders are honed to the correct interference clearance for the new liners. High quality cast iron liners (ductile iron is a very high performance option) are then shrunk in, and final bored to the required running clearance for the modified Wiseco forged pistons. The final part of the big bore process is the final bore using again a SmedSpeed designed and manufactured torque plate system. A torque plate simulates the installed cylinder in the engine, and removes small variations in bore running clearance as a result of cylinder movement.

750cc big bore engine

These days I almost never build 650 engines, the only 650 pistons available are cast pattern pistons which are all Taiwanese made. I know of people who have had no issues, but one thing's for sure, they are unlikely to be the same quality as OEM parts. I use pistons from Wiseco for 750 447 type engines, and either Wossner or JE for 750 533 type engines. 

750cc big bore engine

The standard Wiseco piston, although of excellent quality is a little heavy, and at 10.25: 1 compression a bit high on compression for 95 octane fuel with mild or stock cams.  The combustion chamber, whilst a step forward from the traditional 90 degree “hemi” of the 60s, is still not overly tolerant of high compression pressure. To prevent any chance of detonation and to lighten the piston, I machine the top of the piston to achieve a better shape for flame travel. This sets the compression ratio at 9.6:1 and lowers the compression pressure to a safe figure. The cylinder is finally surface ground to ensure a perfectly flat cylinder surface. I have sent pistons for ceramic coating, this process  increases the cylinder pressure without increasing the compression ratio. It also reduces heat transfer to the piston crown and then the oil.  Included in the Smedspeed kits  is a re-usable copper head gasket, which gives better heat transfer to the cylinder, so aiding cooler running.

The piston modification is a common mod, pioneered many years ago by Joe Minton in his highly regarded “Building a better twin”. The finished piston looks very similar to the piston from a XR750, the XS650/750 major flat track opponent.

750cc big bore engine

The XS650 big bore conversion is of particular interest to anyone whose cylinders have worn past the maximum oversize. Larger capacity increases are possible, 840cc and 880cc are common. These are very high torque engines and helped Martin Guilford to win the pre-1984 twin shock chairs championship in 2016 on an 840. The racing has taught us how to avoid explosions on these big engines, once only reserved for racing. It's quite a lot of detailed work to build a reliable 880 engine, BUT the fun is worth the effort!

Contact me for more details

These are experimental parts intended for racing and as such no warranty is given or implied. If you decide to buy it is on the understanding that you assume complete responsibility for any possible injury, loss or damage that may occur as a result of using these parts and that you accept these conditions of sale prior to purchase.